It is known that, in the manufacture of present day alkaline batteries of the manganese dioxide/zinc type, electrolytic manganese dioxide is the preferred material for use as the cathodic (or depolarizing) reactant. This preference primarily is due to the ability of electrolytic manganese dioxide to provide batteries having significantly improved discharge capacities compared to batteries produced from naturally occurring or chemically produced manganese dioxides.
It also is known that the degree to which electrolytic manganese dioxide can improve the discharge capacities of such batteries depends, to a large extent, on various operating conditions employed in the electrolysis process used in the manufacture of this material. Among the more important operating conditions (or deposition parameters) effecting the degree to which electrolytic manganese dioxide can improve the discharge capacities of such batteries are the purity of the electrolyte, the composition of the electrolyte, temperatures, current densities, the type of material used for anodes and the like employed in the manufacture of the electrolytic manganese dioxide.
In addition to the improvements attainable through optimization of the above noted operating conditions, further improvements in battery discharge capacities reportedly have been accomplished by subjecting electrolytic manganese dioxide to various post production treatments. One such post-treatment is described in Japanese Kokai No. 60-96531 (May 30, 1985). The post-treatment described in this application comprises heating electrolytic manganese dioxide at a temperature between 100.degree. C. and 230.degree. C. Such heating is disclosed as being effective to remove both surface as well as chemically bound water from this material. This removal is reported to result in an electrolytic manganese dioxide having discharge characteristics of longer duration. A further literature article on the thermal post-treatment of electrolytic manganese dioxide and its effect on the discharge capacities thereof is that of J. Koshiba and Yo Karogi in The 3rd Battery Material Svmoosium, Honolulu, 1987, Extended Abstract No. 1.6, IBA Publishers, 1987, pp 21-22.
Another post-treatment that has been proposed for electrolytic manganese dioxide is the method described in Japanese Kokai No. 57-42542 (March 10, 1982). The method described in this application comprises leaching electrolytic manganese dioxide in an alkaline potassium manganate solution at elevated temperatures, e.g. 120 C. The advantage stated to be gained by this post-treatment procedure is an alkaline battery exhibiting a higher open circuit voltage and minimal fluctuation. Good shelf life is another advantage, which it is proposed, would be expected.
It is apparent from the above that, to remain competitive, electrolytic manganese dioxide manufacturers must be able to produce the highest quality product and in a most economical manner. Thus, new processes of manufacture or post-treatments capable of providing such high quality in an economical manner would, by definition, constitute a significant contribution to this field. The present invention provides such a contribution.